1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device that can discriminate volatilization of a memory cell, and also to a semiconductor device which the nonvolatile semiconductor storage device is built in.
2. Description of Related Art
In a nonvolatile semiconductor storage device in which a data can be electrically written, it is necessary to maintain the charge in a memory cell so as to hold the written data therein. Owing to a characteristic defect caused in the production procedure for a nonvolatile semiconductor storage device, however, the data written in a memory cell can be volatilized if the charge of the memory cell cannot be maintained for a time sufficient for the usage. As a result, the written data is varied to cause the malfunction of a program to be executed.
Such a characteristic defect can be avoided by a sufficient accelerated test performed after data writing in a memory cell. In order to shorten the development period for a program and to simplify program modification, however, a data is written not at a semiconductor manufacturer where an accelerated test can be performed but at a program development division where an accelerated test device is not available.
A technique for preventing the malfunction caused by such a characteristic defect in a memory cell is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 64-88998 (1989), 1-300499 (1989) and 62-229597 (1987). In this technique, the characteristic of a sense amplifier is varied so as to make the verification criterion for a data written in a memory cell more strict by increasing one of the input potentials to the sense amplifier when performing the verification of the written data.
The sense amplifier in the aforementioned conventional nonvolatile semiconductor storage device has a sense characteristic as is shown in FIG. 1. In the graph of FIG. 1, the abscissa indicates a power voltage and the ordinate indicates a threshold voltage V.sub.th of a memory cell after writing a data therein, where the sense characteristic is indicated as a straight line SL going up in the right. In the sense characteristic indicated with the straight line SL, therefore, when the power voltage is V.sub.0, the threshold voltage V.sub.th of the memory cell after data writing is V.sub.th2, and the data in the memory cell can be accurately read. In the case where the threshold voltage V.sub.th is decreased to be a voltage V.sub.th1 due to volatilization of the memory cell, the data cannot be read from the memory cell, thereby causing misread.
Further, the nonvolatile semiconductor storage devices disclosed in the aforementioned Japanese Patent Application Laid-Open Nos. 64-88998 (1989), 1-300499 (1989) and 62-29597 (1987) do not perform pseudo read of the data in a memory cell before reading the data from the memory cell. Therefore, the volatilization of the memory cell cannot be discriminated during the usage of the storage.